Conventionally, a low electrically conductive material, such as silica, is compounded in rubber of a tire in order to improve rolling resistance of the tire. This improves the rolling resistance of the tire, but on the other hand increases an electrical resistance value of tread rubber, thus causing a problem that static electricity generated on a vehicle is hardly discharged to a road surface. Then, a method has been employed, in which one sheet of tread rubber layer made by compounding conductive rubber in the tread rubber is applied to a forming drum, thereby manufacturing a tire, and in this method the electrical resistance value of the tread rubber layer is measured before forming. In order to increase the accuracy of measurement of the electrical resistance value, the number of times of measurement needs to be increased, and therefore the working efficiency in the measurement is poor.
In recent years, in place of the above-described manufacturing method of tires, a method of forming the tread by laminating a ribbon-shaped tread rubber onto the molding drum is used, and a tire is produced, which secures electrical conductivity without deteriorating the rolling resistance by laminating a ribbon-shaped tread rubber composed of a low electrically conductive rubber onto an area except the middle portion in the width direction of the tread part and the side face of the tread part and then by laminating a high electrically conductive rubber ribbon onto the middle part and the side face (see Patent Document 1).
However, in this tire with the high electrically conductive rubber ribbon laminated thereon, it is impossible to measure the electrical resistance value in a manner similar to that of the conventional tire manufacturing method of applying the tread rubber layer onto the molding drum. This is because in the conventional tire manufacturing method a place to be measured is one sheet of tread rubber layer while in the recent tire manufacturing method an elongated high electrically conductive ribbon is the target to be measured, and therefore with the conventional measuring method, the number of measured places becomes huge, thus decreasing the working efficiency and also making it difficult to increase the accuracy of measurement.
Moreover, other known conventional electrical resistance measuring devices of tires include the one in which an electrical resistance measuring device is attached to a pair of first and second probes, which are to be pressed against the bead part and tread part of a tire, via electric cords (see Patent Document 2). In this electrical resistance measuring device, when measuring the electrical resistance value of a product tire, the following work is repeated to increase the accuracy of measurement; that is, the pair of first and second probes is abutted against a tire and the electrical resistance value is measured, and thereafter the both probes are detached from the tire and then the tire is rotated slightly at a low speed, and subsequently the rotation is stopped and the both probes are abutted against the tire and the measurement is made. However, this measurement work takes a long time.
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-96402
[Patent Document 2] Japanese Patent Application Laid-open No. 2000-9771